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• W2070394213 abstract "Speciation depends on the establishment of reproductive barriers that allow populations to diverge from each other. Such divergence may involve protein sequence, copy number, or expression changes that are predicted to result in dosage-dependent effects [1Veitia R.A. Paralogs in polyploids: One for all and all for one.Plant Cell. 2005; 17: 4-11Crossref PubMed Scopus (60) Google Scholar, 2Dilkes B.P. Comai L. A differential dosage hypothesis for parental effects in seed development.Plant Cell. 2004; 16: 3174-3180Crossref PubMed Scopus (105) Google Scholar, 3Haig D. Westoby M. Genomic imprinting in endosperm—its effect on seed development in crosses between species, and between different ploidies of the same species, and its implications for the evolution of apomixis.Philos. Trans. R. Soc. Lond. B. Biol. Sci. 1991; 333: 1-13Crossref Google Scholar]. In plants, such as Arabidopsis thaliana and A. arenosa, postzygotic species barriers often affect seed abortion [4Brink R.A. Cooper D.C. The endosperm in seed development.Bot. Rev. 1947; 13: 423-541Crossref Scopus (172) Google Scholar, 5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar], and hybrid failure resembles that of interploidy crosses where the paternal genome is in excess [3Haig D. Westoby M. Genomic imprinting in endosperm—its effect on seed development in crosses between species, and between different ploidies of the same species, and its implications for the evolution of apomixis.Philos. Trans. R. Soc. Lond. B. Biol. Sci. 1991; 333: 1-13Crossref Google Scholar, 5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar, 6Scott R.J. Spielman M. Bailey J. Dickinson H.G. Parent-of-origin effects on seed development in Arabidopsis thaliana.Development. 1998; 125: 3329-3341PubMed Google Scholar]. We used this species pair to explore the relationship between hybrid incompatibility and gene silencing. In incompatible crosses, the normally silenced and heterochromatic element ATHILA was expressed from the paternal, but not maternal, chromosomes. Three Polycomb-regulated genes; PHERES1, MEIDOS, and MEDEA, were also induced. At PHERES1, maternal imprinting of the promoter was disrupted, and paternal imprinting of MEDEA appeared to be lost. The rate of hybrid seed lethality was sensitive to parental genome dosage, and gene activation was proportional to the dosage of parental genomes. A causal link was established between PHE1 and hybrid seed failure; a transposon-induced disruption of PHE1 significantly improved fertility. We propose that the dosage-dependent regulation of chromatin could be a universal phenomenon affecting lethality in interspecies hybrids. Speciation depends on the establishment of reproductive barriers that allow populations to diverge from each other. Such divergence may involve protein sequence, copy number, or expression changes that are predicted to result in dosage-dependent effects [1Veitia R.A. Paralogs in polyploids: One for all and all for one.Plant Cell. 2005; 17: 4-11Crossref PubMed Scopus (60) Google Scholar, 2Dilkes B.P. Comai L. A differential dosage hypothesis for parental effects in seed development.Plant Cell. 2004; 16: 3174-3180Crossref PubMed Scopus (105) Google Scholar, 3Haig D. Westoby M. Genomic imprinting in endosperm—its effect on seed development in crosses between species, and between different ploidies of the same species, and its implications for the evolution of apomixis.Philos. Trans. R. Soc. Lond. B. Biol. Sci. 1991; 333: 1-13Crossref Google Scholar]. In plants, such as Arabidopsis thaliana and A. arenosa, postzygotic species barriers often affect seed abortion [4Brink R.A. Cooper D.C. The endosperm in seed development.Bot. Rev. 1947; 13: 423-541Crossref Scopus (172) Google Scholar, 5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar], and hybrid failure resembles that of interploidy crosses where the paternal genome is in excess [3Haig D. Westoby M. Genomic imprinting in endosperm—its effect on seed development in crosses between species, and between different ploidies of the same species, and its implications for the evolution of apomixis.Philos. Trans. R. Soc. Lond. B. Biol. Sci. 1991; 333: 1-13Crossref Google Scholar, 5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar, 6Scott R.J. Spielman M. Bailey J. Dickinson H.G. Parent-of-origin effects on seed development in Arabidopsis thaliana.Development. 1998; 125: 3329-3341PubMed Google Scholar]. We used this species pair to explore the relationship between hybrid incompatibility and gene silencing. In incompatible crosses, the normally silenced and heterochromatic element ATHILA was expressed from the paternal, but not maternal, chromosomes. Three Polycomb-regulated genes; PHERES1, MEIDOS, and MEDEA, were also induced. At PHERES1, maternal imprinting of the promoter was disrupted, and paternal imprinting of MEDEA appeared to be lost. The rate of hybrid seed lethality was sensitive to parental genome dosage, and gene activation was proportional to the dosage of parental genomes. A causal link was established between PHE1 and hybrid seed failure; a transposon-induced disruption of PHE1 significantly improved fertility. We propose that the dosage-dependent regulation of chromatin could be a universal phenomenon affecting lethality in interspecies hybrids. Arabidopsis thaliana and A. arenosa are two closely related species (estimated divergence time is 3.8–5.8 million years ago [7Kuittinen H. Aguade M. Nucleotide variation at the CHALCONE ISOMERASE locus in Arabidopsis thaliana.Genetics. 2000; 155: 863-872PubMed Google Scholar, 8Koch M.A. Haubold B. Mitchell-Olds T. Comparative evolutionary analysis of chalcone synthase and alcohol dehydrogenase loci in Arabidopsis, Arabis, and related genera (Brassicaceae).Mol. Biol. Evol. 2000; 17: 1483-1498Crossref PubMed Scopus (664) Google Scholar]) that have hybridized in nature to form the allotetraploid species A. suecica[9Kamm A. Galasso I. Schmidt T. Heslop-Harrison J.S. Analysis of a repetitive DNA family from Arabidopsis arenosa and relationships between Arabidopsis species.Plant Mol. Biol. 1995; 27: 853-862Crossref PubMed Scopus (113) Google Scholar]. A. thaliana (At) ovules are readily fertilized by pollen from A. arenosa (Aa), but approximately 95% of the resulting seeds abort upon fusion of gametes of equal ploidy [10Comai L. Tyagi A.P. Winter K. Holmes-Davis R. Reynolds S.H. Stevens Y. Byers B. Phenotypic instability and rapid gene silencing in newly formed Arabidopsis allotetraploids.Plant Cell. 2000; 12: 1551-1568PubMed Google Scholar]. The reciprocal, Aa × At, cross cannot be made [5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar, 10Comai L. Tyagi A.P. Winter K. Holmes-Davis R. Reynolds S.H. Stevens Y. Byers B. Phenotypic instability and rapid gene silencing in newly formed Arabidopsis allotetraploids.Plant Cell. 2000; 12: 1551-1568PubMed Google Scholar] because A. thaliana pollen cannot germinate on the stigmatic surface of the self-incompatible A. arenosa. This is a well-established property of self-incompatible species [11Tiffin P. Olson M.S. Moyle L.C. Asymmetrical crossing barriers in angiosperms.Proc. R. Soc. Lond. B. Biol. Sci. 2001; 268: 861-867Crossref Scopus (225) Google Scholar], and efforts to overcome this crossing barrier have been unsuccessful ([5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar] and our unpublished observation). Hybrid seed failure in the At × Aa cross is characterized by endosperm overgrowth and arrested or altered embryo development. This feature is shared with many interspecies crosses, as well as with intraspecific crosses in which the father has higher ploidy than the mother [5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar, 6Scott R.J. Spielman M. Bailey J. Dickinson H.G. Parent-of-origin effects on seed development in Arabidopsis thaliana.Development. 1998; 125: 3329-3341PubMed Google Scholar]. Bushell et al. observed that, as is true for interploidy crosses [12Adams S. Vinkenoog R. Spielman M. Dickinson H.G. Scott R.J. Parent-of-origin effects on seed development in Arabidopsis thaliana require DNA methylation.Development. 2000; 127: 2493-2502PubMed Google Scholar], demethylating the A. thaliana genome decreased hybrid seed viability [5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar]. Their interpretation was that DNA methylation was required for imprinting A. thaliana genes required for hybrid viability. Crosses between diploid (2x) A. thaliana and tetraploid (4x) A. arenosa, which increase the relative dosage of paternal A. arenosa contributions, also resulted in complete sterility [5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar, 10Comai L. Tyagi A.P. Winter K. Holmes-Davis R. Reynolds S.H. Stevens Y. Byers B. Phenotypic instability and rapid gene silencing in newly formed Arabidopsis allotetraploids.Plant Cell. 2000; 12: 1551-1568PubMed Google Scholar]. These observations are reminiscent of the dosage dependence of interspecies fertility in cereals, potatoes, and tree frogs. In these cases, one can achieve optimal cross fertility by increasing the ploidy of one species above that of the other [13Nishiyama I. Yabuno T. Casual relationships between the polar nuclei in double fertilization and interspecific cross-incompatibility.Cytologia (Tokyo). 1978; 43: 453-466Crossref Scopus (61) Google Scholar, 14Johnston S.A. Hanneman R.E.J. Support of the endosperm balance number hypothesis utilizing some tuber-bearing Solanum species.Am. Potato J. 1980; 57: 7-14Crossref Scopus (82) Google Scholar, 15Bogart J.P. Evolutionary implications of polyploidy in amphibians and reptiles.Basic Life Sci. 1980; 13: 341-378Google Scholar, 16Mable B.K. Bogart J.P. Hybridization between tetraploid and diploid species of treefrogs (Genus Hyla).J. Hered. 1995; 86: 432-440PubMed Google Scholar]. To test the possibility that creating a ploidy imbalance favoring maternal dosage (4x × 2x) could improve hybrid seed fertility, we acquired a wild-collected diploid accession of A. arenosa. Hybrid crosses at all ploidy levels, including 2x At × 2x Aa and 4x At × 2x Aa (A. thaliana ecotype Ler was used in all crosses), were performed (Table 1). Incompatibility in the At × Aa cross was affected by parental genomic dosage. Maternal genomic excess (4x × 2x) strongly suppressed incompatibility in Arabidopsis hybrids, producing many more live seed than crosses of equal ploidy (p < 0.0001). Similar to findings of previous reports, crosses between A. thaliana and A. arenosa of the same ploidy (2x × 2x and 4x × 4x) resulted in more than 90% seed death, and paternal genomic excess (2x × 4x) was completely lethal [5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar, 10Comai L. Tyagi A.P. Winter K. Holmes-Davis R. Reynolds S.H. Stevens Y. Byers B. Phenotypic instability and rapid gene silencing in newly formed Arabidopsis allotetraploids.Plant Cell. 2000; 12: 1551-1568PubMed Google Scholar]. The strong effect of the relative dosage of maternal and paternal genomes on seed viability demonstrates that factors contributed differentially by the two parents affect viability in the hybrid offspring [2Dilkes B.P. Comai L. A differential dosage hypothesis for parental effects in seed development.Plant Cell. 2004; 16: 3174-3180Crossref PubMed Scopus (105) Google Scholar, 3Haig D. Westoby M. Genomic imprinting in endosperm—its effect on seed development in crosses between species, and between different ploidies of the same species, and its implications for the evolution of apomixis.Philos. Trans. R. Soc. Lond. B. Biol. Sci. 1991; 333: 1-13Crossref Google Scholar, 16Mable B.K. Bogart J.P. Hybridization between tetraploid and diploid species of treefrogs (Genus Hyla).J. Hered. 1995; 86: 432-440PubMed Google Scholar].Table 1Hybrid Arabidopsis Crosses Are Affected by Parental Genome Dosage and Maternal PHE1 Genotypep Value of Wild-Type ComparisonsidCross TypeSeed Phenotypenp Value of Wild-Type versus phe1 CrossesAt × AaPlumpaPlump: Seeds that appear regular, are well filled by the embryo, and display the typical tan color of mature Arabidopsis seeds.GreenbGreen: Relatively full, abnormal green seeds.ViviparouscViviparous: Seed containing an embryo that has germinated precociously within the silique.DeaddDead: Very small, dark-brown, and shriveled seeds that contain no discernible embryo.a2 wt × 400.6099.4833a versus c; <0.0001ep values for the null hypothesis of no difference in seed distribution into different phenotypic classes. Data were binned into [dead] and [plump/green/viviparous], and comparisons were made by Fisher's Exact test.b2 phe1 × 400.8099.2654a versus b; 0.7564ep values for the null hypothesis of no difference in seed distribution into different phenotypic classes. Data were binned into [dead] and [plump/green/viviparous], and comparisons were made by Fisher's Exact test.c2 wt × 212.71.694.7880c versus e; 0.0833fp values for the null hypothesis of no difference in seed distribution into different phenotypic classes. Comparisons were performed by chi-square contingency tables for two treatments by four outcomes (plump, green, viviparous, and dead). The p value for [2 wt × 2] versus [4 wt × 2] is <0.0001, and that for [4 wt × 4] versus [4 wt × 2] is <0.0001.d2 phe1 × 20.54.52.492.5925c versus d; 0.0709fp values for the null hypothesis of no difference in seed distribution into different phenotypic classes. Comparisons were performed by chi-square contingency tables for two treatments by four outcomes (plump, green, viviparous, and dead). The p value for [2 wt × 2] versus [4 wt × 2] is <0.0001, and that for [4 wt × 4] versus [4 wt × 2] is <0.0001.e4 wt × 41.24.32.292.3582e versus g; <0.0001fp values for the null hypothesis of no difference in seed distribution into different phenotypic classes. Comparisons were performed by chi-square contingency tables for two treatments by four outcomes (plump, green, viviparous, and dead). The p value for [2 wt × 2] versus [4 wt × 2] is <0.0001, and that for [4 wt × 4] versus [4 wt × 2] is <0.0001.f4 phe1 × 44.312.37.875.6681e versus f; <0.0001fp values for the null hypothesis of no difference in seed distribution into different phenotypic classes. Comparisons were performed by chi-square contingency tables for two treatments by four outcomes (plump, green, viviparous, and dead). The p value for [2 wt × 2] versus [4 wt × 2] is <0.0001, and that for [4 wt × 4] versus [4 wt × 2] is <0.0001.g4 wt × 268.33.96.721461h4 phe1 × 284.71.31.612.3673g versus h; <0.0001fp values for the null hypothesis of no difference in seed distribution into different phenotypic classes. Comparisons were performed by chi-square contingency tables for two treatments by four outcomes (plump, green, viviparous, and dead). The p value for [2 wt × 2] versus [4 wt × 2] is <0.0001, and that for [4 wt × 4] versus [4 wt × 2] is <0.0001.Abbreviations are as follows: wt, wild-type; At, A. thaliana; and Aa, A. arenosa.a Plump: Seeds that appear regular, are well filled by the embryo, and display the typical tan color of mature Arabidopsis seeds.b Green: Relatively full, abnormal green seeds.c Viviparous: Seed containing an embryo that has germinated precociously within the silique.d Dead: Very small, dark-brown, and shriveled seeds that contain no discernible embryo.e p values for the null hypothesis of no difference in seed distribution into different phenotypic classes. Data were binned into [dead] and [plump/green/viviparous], and comparisons were made by Fisher's Exact test.f p values for the null hypothesis of no difference in seed distribution into different phenotypic classes. Comparisons were performed by chi-square contingency tables for two treatments by four outcomes (plump, green, viviparous, and dead). The p value for [2 wt × 2] versus [4 wt × 2] is <0.0001, and that for [4 wt × 4] versus [4 wt × 2] is <0.0001. Open table in a new tab Abbreviations are as follows: wt, wild-type; At, A. thaliana; and Aa, A. arenosa. The observation that unequal combinations of parental genomes produce the best outcome is consistent with decades-old observations in oats and potatoes [13Nishiyama I. Yabuno T. Casual relationships between the polar nuclei in double fertilization and interspecific cross-incompatibility.Cytologia (Tokyo). 1978; 43: 453-466Crossref Scopus (61) Google Scholar, 14Johnston S.A. Hanneman R.E.J. Support of the endosperm balance number hypothesis utilizing some tuber-bearing Solanum species.Am. Potato J. 1980; 57: 7-14Crossref Scopus (82) Google Scholar]. In the case of the potato, endosperm failure is thought to underlie the incompatibility response and prompted the formulation of the Endosperm Balance Number (EBN) theory. This theory states that the outcome of interspecific matings is not determined directly by the parental ploidy level but by the relative quantities of regulatory factors contributed by the parents. Modulating parental dosage can similarly affect hybrid viability in tree frogs, genus Hyla, and interspecific crosses are more likely to produce viable offspring when the female is in genomic excess [15Bogart J.P. Evolutionary implications of polyploidy in amphibians and reptiles.Basic Life Sci. 1980; 13: 341-378Google Scholar, 16Mable B.K. Bogart J.P. Hybridization between tetraploid and diploid species of treefrogs (Genus Hyla).J. Hered. 1995; 86: 432-440PubMed Google Scholar]. Hybrid embryos from incompatible 2x × 2x crosses die at the gastrula stage, which is the time when the paternal genome becomes activated [17Briggs R. The experimental production and development of triploid frog embryos.J. Exp. Zool. 1947; 106: 237-266Crossref PubMed Scopus (20) Google Scholar]. It was suggested that the extra dosage of developmental factors provided by tetraploid females overrides incompatible gene interactions [16Mable B.K. Bogart J.P. Hybridization between tetraploid and diploid species of treefrogs (Genus Hyla).J. Hered. 1995; 86: 432-440PubMed Google Scholar] as might be caused by Dobzhansky-Muller (D-M) incompatibility [1Veitia R.A. Paralogs in polyploids: One for all and all for one.Plant Cell. 2005; 17: 4-11Crossref PubMed Scopus (60) Google Scholar, 18Dobzhansky T. Studies on hybrid sterility. II. Localization of sterility factors in Drosophila pseudoobscura hybrids.Genetics. 1936; 21: 113-135PubMed Google Scholar, 19Muller H.J. Reversibility in evolution considered from the standpoint of evolutionary genetics.Biol. Rev. Camb. Philos. Soc. 1939; 14: 261-280Crossref Scopus (127) Google Scholar]. Another possible interpretation is that the maternal cell contributes essential factors for activating and regulating the paternal genome [2Dilkes B.P. Comai L. A differential dosage hypothesis for parental effects in seed development.Plant Cell. 2004; 16: 3174-3180Crossref PubMed Scopus (105) Google Scholar]. An insufficient dosage of maternal regulators relative to paternal targets could lead to regulatory breakdown, as is the case for hybrid dysgenesis in Drosophila[20Jensen S. Gassama M.P. Heidmann T. Taming of transposable elements by homology-dependent gene silencing.Nat. Genet. 1999; 21: 209-212Crossref PubMed Scopus (173) Google Scholar, 21Blumenstiel J.P. Hartl D.L. Evidence for maternally transmitted small interfering RNA in the repression of transposition in Drosophila virilis.Proc. Natl. Acad. Sci. USA. 2005; 102: 15965-15970Crossref PubMed Scopus (91) Google Scholar]. Molecular expression data are not yet available in either the potato or tree frog hybrid, and interploidy crosses have not been exploited for this purpose in Drosophila. The Arabidopsis interspecific cross provides a tractable model system where the molecular mechanism of dosage-sensitive incompatibility can be explored. We asked whether mating failure is associated with regulatory alterations. Chromatin state, in particular, is known to be sensitive to the dosage of regulatory factors [22Schotta G. Ebert A. Dorn R. Reuter G. Position-effect variegation and the genetic dissection of chromatin regulation in Drosophila.Semin. Cell Dev. Biol. 2003; 14: 67-75Crossref PubMed Scopus (164) Google Scholar]. It has been hypothesized that genomic imbalance could affect heterochromatin maintenance in the hybrid crosses [2Dilkes B.P. Comai L. A differential dosage hypothesis for parental effects in seed development.Plant Cell. 2004; 16: 3174-3180Crossref PubMed Scopus (105) Google Scholar, 23Riddle N.C. Birchler J.A. Effects of reunited diverged regulatory hierarchies in allopolyploids and species hybrids.Trends Genet. 2003; 19: 597-600Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar, 24Birchler J.A. Riddle N.C. Auger D.L. Veitia R.A. Dosage balance in gene regulation: biological implications.Trends Genet. 2005; 21: 219-226Abstract Full Text Full Text PDF PubMed Scopus (267) Google Scholar]. To test this, we investigated the expression of heterochromatic repeats during hybrid seed development. The ATHILA retrotransposon is the predominant pericentromeric element in Arabidopsis[25Pelissier T. Tutois S. Tourmente S. Deragon J.M. Picard G. DNA regions flanking the major Arabidopsis thaliana satellite are principally enriched in Athila retroelement sequences.Genetica. 1996; 97: 141-151Crossref PubMed Google Scholar]. Using degenerate primers designed to amplify a segment from ATHILA elements, we found very low levels of ATHILA expression in RNA isolated from A. arenosa siliques (the seed pods) but not A. thaliana self-crosses. ATHILA expression was strongly induced in hybrid siliques (Figure 1A) and could be detected as early as 1 day after pollination (data not shown). This induction was sensitive to genomic dosage and paralleled the changes in hybrid viability. The lowest expression was found in the most compatible (4x × 2x) cross (Figure 1B). In addition to ATHILA, we surveyed the expression of five other transposable elements by RT-PCR (Ta3, Tar17, CACTA, MULE, and SUNFISH). None of these elements showed transcriptional activation in the hybrid cross (data not shown), suggesting that upregulation of transposable elements is not a general phenomenon in A. thaliana × A. arenosa crosses. The induction of ATHILA in hybrid seed resembles hybrid dysgenesis in Drosophila melanogaster. In crosses between wild and laboratory strains of Drosophila, male-derived transposable elements were activated in hybrid progeny [26Bregliano J.C. Picard G. Bucheton A. Pelisson A. Lavige J.M. L'Heritier P. Hybrid dysgenesis in Drosophila melanogaster.Science. 1980; 207: 606-611Crossref PubMed Scopus (104) Google Scholar]. This happened because maternally contributed repressive small interfering RNAs (siRNAs) were unable to repress paternally derived transposons [20Jensen S. Gassama M.P. Heidmann T. Taming of transposable elements by homology-dependent gene silencing.Nat. Genet. 1999; 21: 209-212Crossref PubMed Scopus (173) Google Scholar, 21Blumenstiel J.P. Hartl D.L. Evidence for maternally transmitted small interfering RNA in the repression of transposition in Drosophila virilis.Proc. Natl. Acad. Sci. USA. 2005; 102: 15965-15970Crossref PubMed Scopus (91) Google Scholar]. Transposon expression in these hybrids was sensitive to maternal siRNA dosage in the egg because of the absence or insufficient copy number of the corresponding transposable element in the maternal genome [20Jensen S. Gassama M.P. Heidmann T. Taming of transposable elements by homology-dependent gene silencing.Nat. Genet. 1999; 21: 209-212Crossref PubMed Scopus (173) Google Scholar, 21Blumenstiel J.P. Hartl D.L. Evidence for maternally transmitted small interfering RNA in the repression of transposition in Drosophila virilis.Proc. Natl. Acad. Sci. USA. 2005; 102: 15965-15970Crossref PubMed Scopus (91) Google Scholar, 26Bregliano J.C. Picard G. Bucheton A. Pelisson A. Lavige J.M. L'Heritier P. Hybrid dysgenesis in Drosophila melanogaster.Science. 1980; 207: 606-611Crossref PubMed Scopus (104) Google Scholar]. We determined the parental origin of ATHILA transcription in hybrid Arabidopsis seed. Cloning and sequence analysis of genomic ATHILA fragments from A. thaliana and A. arenosa demonstrated two species-specific clades of ATHILA elements (Figure 1C; also Figure S2 in the Supplemental Data available with this article online). All of the 25 cloned cDNA fragments amplified from developing hybrid siliques were derived from the paternal A. arenosa genome. The degenerate primers used to amplify ATHILA show a slight preference for the A. arenosa elements (data not shown), potentially introducing bias against maternally derived transcripts during PCR amplification. Primers were designed to specifically amplify either A. thaliana or A. arenosa ATHILA elements (genomic DNA PCR in Figure 1D). RT-PCR with primers specific to A. arenosa ATHILA confirmed the upregulation of these elements in hybrid crosses. RT-PCR with the A. thaliana-specific primers produced no signal in interspecific crosses (Figure 1D), demonstrating that maternal A. thaliana ATHILA are not activated in the hybrid cross. Thus, similar to the situation in Drosophila, the observed activation of ATHILA in interspecific crosses involved derepression of paternally encoded elements. The dosage sensitivity evident from our results suggests molecular explanations similar to the Drosophila hybrid dysgenesis phenomenon. The divergence between A. thaliana and A. arenosa ATHILA elements might underlie functional differences resulting in paternal-specific activation. In addition, the A. arenosa genome is roughly 30% larger than that of A. thaliana, and Southern-blot hybridization indicates that it contains more ATHILA elements than A. thaliana (data not shown). It is possible that either the lower copy number of ATHILA or a decreased competence for expression in A. thaliana, results in insufficient production of siRNA. This could explain the derepression of ATHILA in the genomically balanced hybrid crosses and quiescence in the maternal-excess hybrid crosses. Alternatively, the sequence divergence between A. thaliana and A. arenosa ATHILA elements might reduce the specificity of the maternally (At) contributed siRNA so that a higher input is required to achieve silencing of paternal elements. Such divergence between transposon targets and siRNA regulators would follow the expectations of the D-M model for incompatibility. If A. thaliana egg sacs are unable to suppress paternal ATHILA activation because of a lack of homologous elements, transgenic A. thaliana lines carrying A. arenosa ATHILA RNAi constructs should show reduced activation of A. arenosa ATHILA in the hybrid cross. Seed development in Arabidopsis is negatively regulated by a polycomb repressive complex (PRC) containing MEDEA (MEA) and FERTILIZATION INDEPENDENT ENDOSPERM (FIE) [27Kohler C. Hennig L. Spillane C. Pien S. Gruissem W. Grossniklaus U. The Polycomb-group protein MEDEA regulates seed development by controlling expression of the MADS-box gene PHERES1.Genes Dev. 2003; 17: 1540-1553Crossref PubMed Scopus (317) Google Scholar, 28Kohler C. Page D.R. Gagliardini V. Grossniklaus U. The Arabidopsis thaliana MEDEA Polycomb group protein controls expression of PHERES1 by parental imprinting.Nat. Genet. 2005; 37: 28-30PubMed Google Scholar]. Interestingly, histone methylation mediated by the Eed gene, orthologous to the Arabidopsis PRC gene FIE, is involved in imprinting a subset of the imprinted loci in Mus musculus[29Lewis A. Mitsuya K. Umlauf D. Smith P. Dean W. Walter J. Higgins M. Feil R. Reik W. Imprinting on distal chromosome 7 in the placenta involves repressive histone methylation independent of DNA methylation.Nat. Genet. 2004; 36: 1291-1295Crossref PubMed Scopus (365) Google Scholar, 30Mager J. Montgomery N.D. de Villena F.P. Magnuson T. Genome imprinting regulated by the mouse Polycomb group protein Eed.Nat. Genet. 2003; 33: 502-507Crossref PubMed Scopus (207) Google Scholar]. Loss of PRC function leads to endosperm overgrowth and seed death, similar to the phenotype of hybrid Arabidopsis crosses [5Bushell C. Spielman M. Scott R.J. The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species.Plant Cell. 2003; 15: 1430-1442Crossref PubMed Scopus (111) Google Scholar, 31Grossniklaus U. Vielle-Calzada J.P. Hoeppner M.A. Gagliano W.B. Maternal control of embryogenesis by MEDEA, a polycomb group gene in Arabidopsis.Science. 1998; 280: 446-450Crossref PubMed Scopus (667) Google S" @default.
• W2070394213 created "2016-06-24" @default.
• W2070394213 creator A5035833387 @default.
• W2070394213 creator A5040057158 @default.
• W2070394213 creator A5043097880 @default.
• W2070394213 date "2006-07-01" @default.
• W2070394213 modified "2023-10-12" @default.
• W2070394213 title "Parent-Dependent Loss of Gene Silencing during Interspecies Hybridization" @default.
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